Hello Sofya!
First things first..
pmonth_gtk2 is not a calendar.
It can display a text file in a GTK2 window. Nothing more.
Support script was included to write down the output
from the pmonth_clear or cal commands. Using markup, etc.
Just some experimental hack at a time.. 
Note..
Later version is named SDCalendar. It uses internal calendar layout
and can run as full-time desktop widget with very low system footprint.
No fancy transparency tricks however, due to XPM colorspace limitations.
On the other hand, any of the OTF and TTF fonts should work.
https://www.forum.puppylinux.com/viewto ... 2&start=10
About the cal program.
Unfortunately not all locales in different Busybox versions are correctly
supported.
And Puppy has /usr/bin/cal as a symlink to /bin/busybox mostly, so..
One could replace it with the regular util-linux version of the cal program.
Below is one from the current ( see date of post ) util-linux-2.38 where
32bit is compiled in tahrpup605, 64bit in fossapup64-9.5, please see the docs
folder also.
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Link:
https://www.kernel.org/pub/linux/utils/util-linux/
Note that autopoint may turn up as a dependency..
To build only the cal program, in source folder type in terminal:
./autogen.sh
./configure --disable-all-programs --enable-cal
make cal
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Would be great to see Your changes to the source too!